As a high efficiency AC/AC power conversion system, a direct conversion type power converter circuit, which is typified by a matrix converter, receives attention. In this circuit, power conversion is realized by a combination of a bidirectional switch. Generally, a semiconductor switching device fails to maintain off-characteristics when a negative drain voltage is applied. Therefore, as shown in FIG. 9, a bidirectional switch is realized by combining two sets of series connection circuits of a diode and a transistor.
The on-characteristics of such a bidirectional switch is schematically shown in FIG. 14A. Because the bidirectional switch is composed of a Schottky diode and an IGBT (Insulated Gate Bipolar Transistor), 3V, which is a sum of 1V by the diode and 2V by the IGBT, is generated as an on-voltage. As a result, a power in the hatched part of the figure is lost as a loss due to the switch.
If the switch part is composed of a FET (Field Effect Transistor), such a loss can be reduced to minimum. However, the FET generally does not have a characteristic to block reverse current, that is, reverse blocking characteristic.
FIG. 10 schematically shows drain I (current)−V (voltage) characteristics of a FET that does not have the reverse blocking characteristic. Even under the gate voltage condition that can turn off when the drain voltage is positive, when the drain voltage becomes negative, the potential of the gate shifts to the positive side in relative thereto. Therefore, a channel opens, and reverse current flows. Further, when a positive gate voltage VG is applied, reverse current increases with an increase in the gate voltage VG. like VG1, VG2. and VG3. 
On the other hand, FIG. 11 schematically shows drain I−V characteristics of a FET that has the reverse blocking characteristic. Even when the drain voltage becomes negative, the reverse blocking characteristic can be maintained regardless of the gate voltage VG (VG1<VG<VG3). With use of such a FET, the bidirectional switch can be configured as shown in FIG. 12. This enables cost reduction by reduction of the number of parts, and miniaturization by reduction of the mounting area. Further, by eliminating the diode, a loss during the on-state can be reduced.
As a structure having the reverse blocking characteristic as shown in FIG. 11, a FET in which a drain electrode makes a Schottky contact is proposed in Patent Literatures 1 and 2. FIG. 13 is a semiconductor apparatus that is disclosed in FIG. 1 of Patent Literature 1. On a semi-insulating substrate 105, a channel layer 101 made of an n-type SiC, layer is formed. On top of that, a source electrode 103 that makes an Ohmic contact and a drain electrode 102 that makes a Schottky contact are formed. Further, a gate electrode 104 that makes a Schottky contact is formed between the both electrodes. In addition, a metal layer 106 is formed on the backside of the semi-insulating substrate 105. In the FET disclosed in Patent Literatures 1 and 2, when a negative voltage is applied to the drain electrode, it becomes the same state as when a reverse bias is applied to the Schottky diode, and drain current is blocked, and thereby the reverse blocking characteristic is realized.